In addition to the normal distillation methods, crystallisation methods are becoming increasingly important. There are various reasons for this. One advantage of crystallisation, for example, is that heat-sensitive substances can be obtained or purified at low temperatures. Also, there is no need for an expensive vacuum installation, as in the case of vacuum distillation. In many cases also, higher purity can be obtained than by distillation.
U.S. Pat. No. 3,272,875 describes a crystalliser with pipes through which a coolant flows and the outer walls of which serve as a crystallisation surface. The coolant circuit is a secondary circuit and conveys liquid coolant from the outlet of a circulating pump to one end of the pipes, then through the pipes and from the other ends of the pipes to a heat exchanger and then back to the circulating pump. The heat exchanger transfers cooling energy from a primary circuit, i.e. the circuit of a cooling system The product circuit leads from a product circulating pump to the outer walls of the pipes at one end of the pipes and then from the other ends of the pipes to a heat exchanger and then back to the product-circulating pump. The heat exchanger in the product circuit receives heat energy from a heat source. When the aforementioned crystalliser is in operation, a layer of crystals forms on the outer wall of the pipes. U.S. Pat. No. 3,272,875 does not give any details as to how the layer of crystals is removed. It is known however, that after the mother liquor has been run off from the product circuit, the layer of crystals can be melted off either by raising the temperature in the cooling circuit or by introducing and circulating previously-obtained product. The heating energy necessary for melting can be supplied via the heat exchangers disposed in the product circuit. U.S. Pat. No. 3,272,875 describes the process on a laboratory scale only. There is no information as to how the method described can be converted to industrial practice. More particularly there is no information about multi-stage crystallisation. It can be seen, however, that the need for a number of heat exchangers will result in expensive apparatus. Owing to the resulting energy losses and the chosen method of operation, the energy requirement will be high, the cooling energy being particularly expensive.
Efforts have long been made to reduce the expense of apparatus and energy required for fractional crystallisation. For example in DE-A-17 69 123 it is proposed to cool the crystalliser directly by the medium in the cooling plant, by withdrawing vapour from the space in the crystalliser jacket. However there is no information as to doing this in practice, and also no directly-cooled crystallisers have become known during the 25 years since U.S. Pat. No. 3,272,875 was published.
In contrast to the crystalliser according to the initially-menticned U.S. Pat. No. 3,272,875, in the crystalliser according to DE-A-1 769 123 the pipes are cooled from the exterior and the liquid mixture is delivered in a film trickling down on to the insides of the pipes. Also in multi-stage crystallisation a crystalliser is not provided for every stage. Instead, multi-stage crystallisation is effected in different cycles in a single crystalliser. To reduce the energy costs further, it is proposed that the heat produced in the condenser of the cooling plant should be partly stored in a heating-medium tank and used later to melt the crystals, via a heat-exchanger in the product circuit. Excess heat is to be discharged in cooling water or in extreme cases via a second cooling plant. This known method has the disadvantage of expensive apparatus and energy losses during energy storage and conversion.
U.S. application Ser. No. 08/566,792, filed Dec. 4, 1995 U.S. Pat. No. 5,700,435, by the inventor here of, the disclosure of which is hereby incorporated by reference, describes a method of separating a substance from a liquid mixture by fractional crystallisation, wherein a layer of crystals is deposited on one side of a wall which is cooled on the other side. Cooling is effected by evaporating a medium, and by adjusting the pressure of the gaseous phase of the medium in the crystalliser, in accordance with the temperature required for crystallisation. On the other hand, the layer of crystals is melted or is caused to sweat by condensing the medium, supplied in the gaseous phase, on the wall. In this operation the pressure of the gaseous phase is controlled in accordance with the temperature required for the purpose at the wall. If at least two crystallisers are present, one crystalliser can be used for crystallisation and the other crystalliser for melting the crystallised substance. Consequently one crystalliser operates as a condenser and another crystalliser operates as an evaporator in a refrigerating machine. In this manner both the cold energy and the waste heat from the refrigerating machine are directly used. This saves considerable energy compared with the known method.
Also, in the method describe in my aforemention application Ser. No. 08/566,792, additional heat energy is supplied if necessary for melting the crystallised substance. This purpose is served by an auxiliary evaporator which can be connected by pipes and valves to one or tore crystallisers requiring the additional heat energy. For example the crystallised substance can be melted more quickly than if the only available energy is from another crystallise. In general, the availability of external heat energy makes the process more flexible. The disadvantage, however, is the expensive apparatus required in the form of piping and liquid separators for the refrigerating medium. There may also be problems through condensation and resulting liquid surges.